This invention relates to the design, synthesis, and application of a new family of aza-ether based compounds which act as anion receptors in non-aqueous battery electrolytes. As a result, the anion receptors of the present invention can be used as additives to enhance the ionic conductivity and cation transference number of non-aqueous battery electrolytes. More specifically, the new family of aza-ether based compounds includes substituted linear aza-ethers, multi-branched aza-ethers, and aza-crown ethers wherein the hydrogens on the N are substituted with an R group. R is an electron withdrawing group such as CF.sub.3 SO.sub.2, CF.sub.3 CO, CN, or SO.sub.2 CN. The electron deficient N centers can complex anions very effectively.
Considerable research has been conducted on the design and synthesis of receptor molecules for the selective complexation of ions. While compounds which serve as cation receptors have been studied widely, information on host molecules for anions has been scarce. For example, U.S. Pat. No. 5,130,211 to Wilkinson, et al. describes an electrochemical cell which has an electrolyte solution containing an organic solvent, an alkali metal salt and at least one sequestering agent. The sequestering agent described by Wilkinson, et al., can be a glyme, crown ether or cryptand. The sequestering agents disclosed by Wilkinson, et al. are useful in complexing with the alkali moiety of the electrolyte salt and thus, act as cation complexing agents.
Dietrich (B. Dietrich, J. Pure and Appl. Chem. 65, 1457 (1993)) has reviewed the field of anion receptors and has pointed out that the requirements for anion receptors are different than those for cation receptors. This is because of the larger size of the anions and the wide variety of shapes encountered in polyatomic anions. At present, two categories of anion receptors have been developed. One includes host molecules that contain positively charged sites, the others are neutrally charged anion receptors. For the first category, the positively charged sites such as ammonium or guanidinium binding sites act as large cations and can only be used in aqueous solutions where the anions are already dissociated. These types of anion receptors cannot be used in non-aqueous electrolytes to increase ion dissociation. In the second category, the anion binding depends on either hydrogen bonding or electron deficient Lewis acid metal centers (Sn, Hg, B, or Si) in organic structures. Although these neutral receptors are useful as selective anion binding sites in both aqueous and non-aqueous solutions, they are not suitable for use in aprotic non-aqueous electrolytes. Receptors which utilize hydrogen bonding will react with lithium or sodium and will thus degrade both the anode and the electrolyte. The Lewis acid type receptors often contain heavy metals and as a result tailoring of a structure to fit various anions is difficult. Furthermore, their electrochemical stability is unknown and the released metals from decomposition would be deleterious in a battery electrolyte. The anion receptors of the present invention are a new type of neutral receptors. The novel feature of these receptors is that the anion binding is accomplished neither through a metal Lewis acid center nor through hydrogen bonding, but through an electron deficient center at N atoms that is induced by the substitution of the amine hydrogens with electron withdrawing groups. The receptors are electrochemically stable and can be easily designed to fit various anion sizes and shapes. Their chemistry is compatible with that of electrodes, salts, and solvents commonly used in non-aqueous battery electrolytes.
Moreover, in the past, inorganic, cost effective salts such as LiCl and LiBr have not been used as electrolyte salts because of their low solubility and conductivity. LiI has been used as an electrolyte to some extent because LiI has higher conductivity than LiCl and LiBr. However, I.sup.- is oxidized above 2.5 volts vs. Li so it could only be used in low voltage primary batteries such as Li/FeS.sub.2.
Accordingly, there is still a need in the art of alkali metal batteries and especially lithium batteries for electrolyte additives which can complex anions, yet are stable in alkali metal and especially lithium batteries. There is also a need in the art of alkali metal batteries to enhance the conductivity of inexpensive and environmentally friendly inorganic salts such as LiCl, LiBr and LiI. In addition, there is a need to increase the transference number of the Li.sup.+ ion. In many non-aqueous electrolytes, in particular polymer electrolytes, the transference number of the Li.sup.+ ion is low. This introduces additional polarization losses in batteries and reduces the utilization of the cathode material.
It is therefore, an object of the present invention to provide a new family of compounds which enhances the conductivity of alkali metal battery electrolytes by complexing with the anion moiety of the electrolyte salt and also by increasing the transference number of the Li.sup.+ ion.
Another object of the present invention is to increase the conductivity of cost effective electrolyte salts such as LiCl, LiBr and LiI.
Another object of the present invention is to provide improved electrochemical cells by use of electrolyte additives.